The present invention relates to a method of centrifugally purifying used mineral oils, especially heavy oils contaminated with particles of dirt and slight amounts of water, by means of a self-emptying separator, with the mixture of solids and liquids being supplied to the separator's centrifuge drum through a central intake and with the purified oil being extracted from its center through a run-off, the separator also having another run-off supplied from a sampler for liquid extracted from the periphery of the drum, with a sensor positioned in the second run-off to determine the consistency of the fluid and with a water run-off valve downstream of the sensor, the liquid being returned to the intake when the water run-off valve is closed.
A method and device of this type are known from German OS No. 3 314 859. The sampler is the known device operates continuously and the liquid extracted from the periphery of the drum and supplied to the second run-off is either returned to the central drum intake or, if a sensor so determines, detoured out of the circulation system through a three-way valve.
As separation commences, oil flows into the second run-off through the run-off channel for the liquid extracted from the periphery of the drum and water accumulates in the drum's solids space. During this phase of the operation the three-way valve is positioned to return the oil to the drum's central intake. As soon as the water accumulated in the solids space arrives at the run-off channel, it is extracted through the channel and through the sampler and supplied to the second run-off. When the sensor, which can be an instrument that measures the electric conductivity of the liquid, determines that water instead of oil is flowing through the second run-off, the three-way valve is repositioned and the water is supplied to a catch point.
It has turned out in practice that a stable water phase cannot be extracted through the second run-off in accordance with the known method because the proportion of water in the intake into the drum is much lower than the run-off capacity of the amount of liquid extracted from the periphery of the drum. Thus, oil is also extracted along with the water, and both components emulsify, preventing separate diversion of the water phase. Emulsification is also promoted because the specific weights of oil and water differ so slightly and because liquid is constantly being extracted from the periphery of the drum through the sampler.